The point for me is a biblical point:
a. does the total bible, to include the NT, require the death penalty
b. does it all the death penalty
c. does it ban the death penalty.
Biblically, "b" is the answer. Therefore, you are able to hold your position and still be biblical. It would be wrong for you, however, to tell a death penalty advocate that they were biblically in error.
You could tell them that given current circumstances they aren't making the best choice.
I read recently that there are those who have been sent to death row who should have been sentenced to a lesser murder charge, and that that is the only way people conclude that we have placed convicts on death row in error.
No one disputes their being murderers.
I would not tell someone who supports the death penalty that it was unBiblical. The problem isn't the Bible, it is all the corrupt prosecutors and judges who are willing to try and convict those whom they may actually believe may be innocent, even if they have to ignore the rule of law in doing so. to to many prosecutors, it is not about justice; it's about winning; and that is wrong.